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Boat Engine Repairs . Solutions for hot or overheating boat engines

By Capt. Douglas Malat

Engine Running Hot or Overheating..

As you know, engine repairs is a broad area to discuss because of so many factors that can cause an Engine to Run hot or Overheat. Let's get started.

For this Article, we'll keep it to Freshwater Cooled Inboard, Inboard/Outboard gasoline engines...but most of the information can also relate to Diesels.

Let's open the hatches and take a look...There are several components (parts) to the Cooling System.

1) Belts and Hoses
2) Saltwater thru-hull fitting
3) Salt water pump (has rubber impeller)
4) Heat Exchanger (Freshwater cooling)
5) Engine Water pump (freshwater)
6) Thermostat
7) Antifreeze

Let's go over each component/part and see what can cause your Engine to Run hot or Overheat. It could be one thing that is causing the problem or a combination.

Engine Repairs step 1: Belts and Hoses:
Sometimes it is as simple as adjusting your fan belts. One fan belt turns your Salt water pump, another turns your Fresh water fan belt should not turn both. The fan belt should sit towards the top of the pulley groove, not sunken within it...a sign that the belt is worn and has used up its useful life. Change any worn fan belt and adjust to tighten to manufacturers specs. It should have about a ½ inch of play when pushing down or up on the fan belt. With new and/or well adjusted fan belts, the pump pulley will now turn more efficiently with no slipping and pump more water through your engine.

Your Cooling System Hoses should not have any leaks or be bloated. They should feel firm so the flow of water in the hose is not deformed or restricted in anyway. Again this brings more volume of water throughout your Engine, cooling it down. Don't forget to check your hose clamps and tighten where necessary.

Note: Loose Fan Belts, distorted/bloated Hoses can cause water flow restrictions. At higher speeds and rpms, the Engine creates more heat and...additional cooling power is needed. If you have slipping fan belts and distorted hoses to lower cooling water flow, you can bet that your Temperature gauge is going to keep rising where you don't want it. This commonly necessitates engine repairs.

Engine Repairs step 2: Salt water thru-hull fitting:
Your Thru-hull Salt water Intake of course should work. The ball valve should move easily and the hose should have a firm feel to it. If the sides squeeze in too easily, the Salt water pump it goes to can collapse the hose to cut off water flow...even if it collapses part way, this restriction will affect your Engine cooling and keep your engine Temperature up. So make sure the hose is good. The Thru-hull fitting on the underside of the boat has a scoop with some grating on it to keep larger objects from getting sucked in. What I usually do if I have an Engine running hot or Overheating, is to test it out and clean it. On smaller boats, you can get underneath to clean the grating. If your Thru-hull fitting ball valve is working properly, I close the Seacock (Ball valve), remove the hose and open the Seacock to see the amount of flow...It should be a gusher (have your wooden plug close by for safety). If the amount of water coming in is questionable, I usually close the Seacock, attach a garden hose to it and turn it on. This is reverse pressure and will clean up that grate underneath somewhat...turn the Seacock on and off a few times to help blow off any growth on it.

Note: You have just checked the water flow thru the Thru-Hull fitting, (hopefully good flow)...checked the hose for firmness that goes to your Salt water pump...Looking good! You're becoming an expert in engine repairs.

Engine Repairs step 3: Salt water pump (has rubber impeller):
Now we get to the Salt water pump that flows the salt water to your Fresh water Heat Exchanger (cools your antifreeze). Check for any leakage externally. Now let's check your rubber impeller...I know you might have done it recently, but read on. Remove the rubber impeller and examine each rubber blade. It should have no cracks in it as you bend them one way then the other. There should be no splits anywhere either. You can compare it to a new one you have onboard...If you don't have a new spare impeller, it is cheap insurance if you do...The rubber impeller should not move separate to the metal should be well attached. Back in the pump itself, there are two from your intake hose (thru-hull fitting) and one going to your Heat Exchanger. Look closely into those holes...some of your past impellers might have lost a blade and lodged in the hole or further down. Inspect carefully and remove any foreign pieces and use a garden hose to water jet anything else would be good to have both hoses detached and make sure the Seacock is off. You also have a beveled piece of brass in the pump to squeeze the blades and create suction. Make sure it's there of course and that it is smooth. If it is torn up, it will do the same to your rubber impeller. Have a good gasket seal on your impeller cover plate (don't want to suck air) and add some grease to your Shaft grease cap. Make sure when you tighten the cap, grease is getting to the shaft. It will lubricate your Shaft and Shaft seal so you will not leak...If the leak persists out of the shaft, a new shaft seal may have to be installed.

Note: With good flow to and out of your Salt water pump we're looking good.

More on engine repairs.

Engine Repairs step 4: Heat Exchanger (Fresh water cooling):
Your Heat Exchanger is generally tubular, having two separate areas within it. One area for the Salt water, water flow and the other area for the Fresh water flow. The Fresh water (water/antifreeze mix) is hot from absorbing the heat within the engine. As the Fresh water flows through the Heat Exchanger, the cool Salt water picks up the Fresh water heat, cooling the Fresh water down so it is ready to go back thru the Engine to pick-up some more heat. The Salt water that is now hot from the Fresh water flows out of the Heat Exchanger into the end of the Exhaust Manifold to cool the Real Hot Exhaust as it leaves the Engine into the exhaust hose to a muffler then overboard (Generally speaking)

Some seaweed can get caught up in here (Salt water side) as well as eel grasses. You can remove the end caps to blow out or jet away any foreign matter. (The end caps generally only open the saltwater side, be sure). You can also disconnect the Salt water hoses and back flush with a garden hose and clear it out that way. A good check for restriction of Salt water flow is to disconnect the saltwater hose coming from the Heat Exchanger To the Exhaust manifold. Where the hose connects, rust seems to squeeze the hole shut. Being gentle, scribe out any loose rust and/or foreign matter that has lodged itself in this area. Easily blow or jet some water thru this area to make sure all is clear.

Note: Generally, all that happens in the Heat Exchanger is that things get caught up in it, restricting flow...all we want is to clear it up so the heat exchanged from the Fresh water to the Salt water happens faster, cooling the Fresh water down even further. Remember to check that exhaust manifold connection...if the salt water gets restricted there, it keeps your fresh water hotter...and we don't want that...we want it to flow through unobstructed!

Note: With this area done correctly, your Fresh water should come out of the Heat Exchanger cooler so it can pick-up more engine heat, keeping your engine cooler.

Engine Repairs step 5: Engine Water pump (freshwater):
Your Engine Water pump is usually an integral part of the engine. It bolts on and off, but still check around the gasket areas for leakage. There is really no serviceable parts and all this pump does is circulate the Freshwater (water/antifreeze mix) around the engine to pick-up heat. It goes back to the Heat exchanger to transfer the heat to the Saltwater, and then back around the engine again. There is a weep hole (small little hole) under and behind the water pump pulley. If you see this leaking at all, replace it. When the Engine is off, see if you can take a flashlight and find the weep hole. Sometimes there will be staining which tells you it does leak...change it, it will only get worse. Also, if the Engine Water pump is many years old, the metal impeller inside can be worn. This will slow down the movement of the Freshwater around the engine. It would be a good idea to change it...If it is that old, its. days are numbered.

Note: The Engine Water pump is a straight forward engine component. No water weeping from the hole, no water stains there, and the pump isn't that old...I think we're good here.

Engine Repairs step 6: Thermostat:
Your Engine Thermostat is what regulates the Engine Temperature. You can get Thermostats at different temperatures, but get the one rated for your Engine.

An old Thermostat will be sluggish, causing the Engine temperature to usually stay high. A sticky Thermostat will cause Overheating. No Thermostat at all will cause the Engine to Run Hot or Overheat, because the water is passing thru too quickly to pick-up any heat. At idle it will be fine (but too cool), but at higher speeds and rpms, the water will be zooming thru...Too Fast. Get a good brand Thermostat installed, rated for the proper temperature and your set.

Note: With the right Thermostat, you can't go wrong. You want your Engine temperature to be regulated at the proper temperature. A little prevention can help to avoid future engine repairs.

Engine Repairs step 7: Antifreeze:
Here the proper mixture of water and Antifreeze is essential. This allows optimum capture of heat (a high transfer rate). Check the Engine Manufacturers specs.

*Final Note: There are other things that affect the Engine temperature such as Engine Ignition Timing, wrong Prop size, clogged or internally cracked Manifolds...etc. But going over these basics areas will either fix or help you narrow down your problem. Plus you get to learn more about your Engine Cooling System!

Happy Boating! Hopefully, this information will assist you in your boat engine repairs.

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